The concentration of an exhaust composition in the emissions from an internal combustion engine has been used as a signal to represent the ratio of the air-fuel mixture in order to control the mixture ratio in the neighborhood of the stoichiometric value in order to reduce the noxious components to a minimum. In multi-cylinder internal combustion engines in which the cylinders are equally divided into two groups for separate emission of exhaust gases, an exhaust composition sensor is provided for each exhaust system because of the difference in the air-fuel ratio between the exhaust systems arising from the possible different mechanical tolerances or workmanship. In such systems, air-fuel ratios of the exhaust systems are separately feedback-controlled, and if one of the exhuast composition sensors should fall, the cylinders associated with the defective sensor will be supplied with a rich or lean mixture depending upon the type of failure. As a result noxious emissions will be produced even though the other exhaust system is functioning properly and drivability is also impaired.